The invention relates to an optical focusing device in an optical information read-out system and, more particularly, to the improvement of a detecting unit of the optical focusing device.
The optical information read-out system, which has intensively developed recently, is used to read out the information recorded on a moving data carrier such as an information disc or tape by a convergent light beam. In such an optical information read-out system, a lightbeam must be focused onto the information recorded surface of the moving data carrier to produce accurately a covergence point thereon, in order to achieve an accurate focusing adjustment and to accurately read out the information from the moving data carrier. The optical information read out system is usually provided with an optical focusing devcie for ensuring the accurate focusing adjustment. U.S. Pat. No. 4,079,247 and the Japanese Patent Publication No. 37722/78 disclose the conventional optical focusing devices of the type in which an astigmatism optical element such as a cylindrical lens and a photosensing unit onto which a light beam passing through the cylindrical lens is projected are combined. The problem involved in the U.S. patent device is that, when the light beam focussed onto the information disc is directed from one track to another track, it is reflected through the optical path different from the previous optical path. Accordingly, the light beam spot is shifted on the photosensing unit, so that an inaccurate focusing adjustment is attained. This problem arises from the arrangement of the photoelectric cells forming the photosensing unit and the locations of the photoelectric cells. The optical device of the Japanese Patent Publication was proposed to solve the incorrect-positioning of the light spot. However, the optical focusing device still involves problems. For example, even though the focusing adjustment is not yet completed, its operation frequently is completed or even though the it is satisfactorily completed, the focusing adjustment operation is still continued, often.